A. Field of the Invention
The present invention relates generally to the field of non-contact tonometry, and more particularly to a non-contact tonometer employing an improved applanation detection system which increases measurement reliability by making the instrument less sensitive to alignment variations.
B. Description of the Prior Art
In the operation of known non-contact tonometers for measuring intraocular pressure (IOP) of a patient's eye, an increasing force air pulse is discharged through a pump-fed nozzle along a test axis toward the eye to deform the cornea from a state of convexity, through an instantaneous state of "applanation" wherein a predetermined area of the cornea is flattened, to a state of concavity; the cornea is then allowed to return to its original convex state under natural forces. Opto-electronic means are used to continuously monitor the corneal deformation and thereby determine the moment of applanation. The interval of time or pump plenum pressure required to achieve applanation is then used as a correlate of IOP.
Heretofore, applanation detection systems used in non-contact tonometers have been telecentric in nature, requiring precise three-dimensional alignment of the discharge nozzle relative to the cornea. For example, the XPERT.RTM. NCT manufactured by Reichert Ophthalmic Instruments, a division of Leica Inc., assignee of the present application, currently employs a telecentric applanation detection system as shown in FIGS. 1A and 1B. An emitter E directs an incident beam of light represented by parallel rays R1, R2, and R3 at an oblique angle toward corneal pole C. A single photo-detector D, which is positioned at a symmetrically equal and opposite angle behind a pinhole aperture A in the focal plane of a collector lens L, receives a small bundle of corneally reflected rays. When the cornea is in its normal convex state, as shown in FIG. 1A, the fan-shaped dispersion of rays R1-R3 results in a weak detection signal. As the cornea is progressively transfigured toward a state of applanation by the fluid pulse, the fan-shaped configuration of dispersed rays closes and the detector signal becomes stronger. When applanation occurs according to FIG. 1B, dispersion of rays R1-R3 is at a minimum, resulting in a peak signal at detector D.
As may be appreciated, the telecentric system described above is extremely sensitive to misalignment of the instrument with respect to the cornea. FIG. 2 includes a plot of a typical detector signal as a function of time in a properly aligned instrument, and FIG. 3 includes a similar plot in a misaligned instrument. The undesirable consequence of misalignment is a false reading of an early signal peak, which will yield an inaccurate IOP measurement that is usually lower than actual IOP. Since an important reason for subjecting the patient to IOP measurement is to detect elevated IOP, an indication of glaucoma, falsely low IOP measurements due to misalignment are a serious and recognized problem. In view of this, a large portion of the cost of a commercially available non-contact tonometers is attributed to the expensive opto-electronic alignment system necessary for precise alignment. In the XPERT.RTM. NCT discussed above, the applanation detection system imposes stringent alignment requirements on the order of .DELTA.X.ltoreq.0.05 mm, .DELTA.Y.ltoreq.0.05 mm, and .DELTA.Z.ltoreq.0.3 mm, where X is side-to-side, Y is vertical, and Z is distance from the patient.
It is further noted that efforts to relax alignment precision requirements by providing a larger pinhole aperture at the detector are not helpful because signal resolution is lost, once again making it impossible to accurately determine the moment of applanation with certainty.